Hydrocarbon fuels such as diesel oil and fuel oil are produced by refining crude petroleum. However, petroleum represents a non-renewable resource. Therefore, researchers have dissolved other ingredients such as water-soluble alcohols like methanol and ethanol in hydrocarbon fuels to reduce petroleum consumption. Although the alcohols dissolved in the hydrocarbon fuels have acceptable combustion characteristics, there is a problem of possible phase separation if the fuel tanks become contaminated with a small amount of water. The so called "water bottoms" have an affinity for the water-soluble alcohol resulting in the water-soluble alcohol dissolving in the aqueous phase and causing phase separation which cannot be tolerated.
Wenzel and Steinmann's U.S. Pat. No. 4,083,698 provided a solution to this problem. Specifically, this patent discloses a clear, low viscosity, stable water-in-oil (W/O) microemulsion fuel composition containing both an ethoxylated non-ionic surfactant and an anionic surfactant. This unique combination of surfactants successfully prevented phase separation of a fuel composition containing water , a water-soluble alcohol and a hydrocarbon fuel. Importantly, these microemulsion fuels such as those in which the hydrocarbon is diesel oil had unusually good stability, especially stability below the freezing point of water.
While these fuel compositions containing both ethoxylated non-ionic surfactants and an anionic surfactant represented an improvement over the prior art, the ethoxylated non-ionic surfactants are not entirely satisfactory. The concept, which is a part of this invention, is that the ethoxylated non-ionic surfactants used in such a large amount according to the teachings of U.S. Pat. No. 4,083,698, result in unsatisfactory combustion of the microemulsion fuel and engine performance. Specifically, the ethylene oxide add-on in the surfactants is like ethylene glycol or could even be a precursor for ethylene glycol during combustion and the concept is that these have poor combustion characteristics. Also, the water content of these fuel compositions is not large enough to affect a reduction in the nitrogen oxides (NOx) exhaust emissions in the absence of a NOx scavenger.
For these reasons improvements in U.S. Pat. No. 4,083,698 are needed which will provide better combustion and engine performance for the microemulsion fuels and simultaneously provide environmentally superior microemulsion fuels as substitutes for 100% petroleum products such as diesel oil and fuel oil.
The oxides of nitrogen (NOx) is a serious pollutant from diesel engines and other internal combustion engines. The high ratio of air to fuel and the high temperatures obtained in the combustion of diesel oil fuel lead to high NOx. However, the high ratio of air to fuel is necessary for complete combustion to occur.
There are two problems on the opposite side of the scale. One is particulate matter which can only be reduced by increasing the degree of combustion of the fuel. The other is NOx which tends to increase as the particulate matter is decreased.
The use of W/O microemulsion fuels is the most meaningful way to obtain the balance of good engine performance and abatement of air pollution for internal combustion engines and heating oil or fuel oil furnaces. The oxygen content of the microemulsion fuel which is generally 13-14% by weight in this invention results in more complete oxidation and therefore, lower particulate matter in the exhaust gases. The water in the microemulsion fuel results in a lower temperature of combustion of the microemulsion fuel which tends toward lower NOx. It has been established that the temperature during combustion is a main factor regarding NOx, the lower the temperature the lower the NOx and vice versa.
The water content of the microemulsion fuel becomes the critical parameter. The more water in the formulation the greater the reduction in NOx. However, unfortunately, the greater the water in the formulation the less the engine power or the higher the brake specific fuel consumption (BSFC). So without any NOx scavengers there must be a compromise between NOx reduction and BSFC which is almost entirely dictated by the water content of the microemulsion. There is considerable evidence that a major reduction in NOx requires a high percentage of water in the formulation without the presence of NOx scavengers. But then this leads not only to a high BSFC but also, more incomplete combustion which could result in higher particulate matter in the exhaust gases.
U.S. Pat. No. 5,004,479 by Schon and Hazbun discloses microemulsion fuels. They use unsaturated fatty acids partially neutralized with a nitrogenous base as the anionic surfactant including the use of ammonia which was used in U.S. Pat. No. 4,083,698 by Wenzel and Steinmann. However, they omit the ethoxylated non-ionic surfactants. Schon and Hazbun made a comprehensive study of the extent of neutralization of the unsaturated fatty acid versus the water up-take exhibited by their drawings of FIGS. 1 to 4. Of particular interest to the present invention is their FIG. 1 where ammonia is the nitrogenous base. FIG. 1 shows optimum water up-take when about 80 mole percent of the unsaturated fatty acid is neutralized with ammonia. In their TABLE 1 they teach that the optimum mol percent of the neutralization of the fatty acid of 80% corresponds to a water uptake of 0.20-0.33 grams of water per gram of diesel oil. Then in their TABLE 2 they teach that for E-315/NH3 (80) the percent water by weight in the microemulsion fuel is 15%. However, freezing of the microemulsion fuel occurs at both temperatures of -20.degree. and -10.degree. C. and there is even turbid or phase separation at 0.degree. C. according to their data. This means that if they want the optimum water content for reducing NOx, their microemulsion fuel will not have satisfactory stability at sub-zero temperatures.
In the present invention, the mole percent of the unsaturated fatty acids neutralized with ammonia is 64%. With reference to Schon and Hazbun's FIG. 1, the uptake of water at 64 mole percent neutralization with ammonia is only 0.013 grams of water per gram of diesel oil. The uptake of water in the present invention is about nine times more than this amount which will be shown in the examples.
The reason for the greater uptake of water in this invention is because of the new and novel use of a non-ethoxylated surfactant and a water-insolube aliphatic alcohol melting below 0.degree. C. such as octanol-1 in combination with the anionic surfactant of the ammonium salt of the unsaturated fatty acids. Furthermore, it will be shown that the low temperature stability of the microemulsion fuel is excellent at -15.degree. C. Based on these results, it is believed that the microemulsion fuels of this present invention represent an improvement in the art over those of Schon and Hazbun.
The second point is that Schon and Hazbun rely primarily on the water and methanol content of the microemulsion to reduce the NOx. In this invention the microemulsion fuel also contains water and methanol like in U.S. Pat. No. 4,083,698 but in addition, the NOx scavengers urea and ethyl carbamate are used to enhance the decrease of the NOx at lower water content of the microemulsion so as to also maintain good engine power or low BSFC and achieve a simultaneous decrease in particulate matter in the exhaust gases as described above.
Peter-Hoblyn and Valentine in U.S. Pat. No. 5,584,894 discloses the use of emulsions of water and diesel oil with catalysts to promote the reduction in NOx. High percentages of water are used which they quoted as 15% to 45% for the preferred range. Many emusifiers are mentioned in this patent including the alkyl amines and hydroxyalkylamines reacted with fatty acids but the reaction of ammonia with unsaturated fatty acids and the use of water-insoluble aliphatic alcohols melting below 0.degree. C. and acetylenic non-ethoxylated surfactants are not mentioned in this patent.
A second point relates to the emulsions referred to in their patent. There is no information on the stability of the emulsions at sub-freezing temperatures. The particle size mentioned in their patent in which "at least 70% of the droplets are below about 5 microns" classifies this as an emulsion not a microemulsion. On the other hand, a microemulsion has an average particle size of about 0.01 micron. This invention like U.S. Pat. No. 4,083,698 and U.S. Pat. No. 5,004,479 deals with microemulsions.
In his patents, U.S. Pat. No. 5,404,841 and U.S. Pat. No. 5,535,841, Valentine teaches that the NOx scavenger urea enhances the decrease in NOx and that it is preferrable to have a NOx scavenger in the emulsion and not rely solely on the water content of the emulsion to reduce the NOx. He also mentions ammonium carbamate as a NOx savenger in addition to many others. He describes the SNCR reducing process (selective non-catalytic) but he also discloses a catalytic process (SCR) for reducing NOx.
Previous patents and literature on water injection systems for exhaust gases teach that urea is an effective NOx scavenger. It is believed that the mechanism involves the formation in part of ammonia from urea with the ammonia actually being the reducing agent for NOx. In fact, aqueous ammonia solutions for the water injection system are very effective in reducing NOx.
Like the patent of Peter-Hoblyn and Valentine, the Valentine patents involve emulsions of large particle size compared to microemulsions with much smaller micelle size as explained above. Again, no information is given on the stability of these emulsions at sub-freezing temperatures. It is stated that the reason for the recirculation line shown in his drawing is "to maintain emulsion stability".
In contrast, the microemulsion of this invention has indefinite storage stability. The appearance, viscosity and flow properties of the microemulsion of this invention is so similar to that of 100% diesel oil that it is difficult to distinguish one another by visual examination. The advantages of the stability of the microemulsions over long storage periods and also at sub-freezing temperatures of this invention and still containing NOx scavengers in contrast to the Valentine emulsions with diesel oil are apparent and considered to represent an improvement in the art. A novel concept of this invention will be described to show that ethyl carbamate is considered to be superior to ammonium carbamate as a NOx scavenger.
The basic idea in this invention regarding a solution to the problems of particulate matter and NOx, is to maintain a comparatively low water content of the microemulsion fuel to promote complete combustion resulting in acceptable BSFC's and low particulate matter and simultaneously achieve a good NOx reduction by means of NOx scavengers present in the formulations of the W/O microemulsion fuels. An important criterion of this invention is that the microemulsion fuels with diesel oil have indefinite storage stability and maintain good stability at sub-freezing temperatures so that there is not any phase separation and that the microemulsion fuel has good fluidity for transport in the fuel lines even at sub-freezing temperatures. It is believed that the concepts and ideas of this invention represent an improvement in the art of microemulsion fuels made with diesel oil and heating oil or fuel oil.